In this subsection, we assume that the perfect CSI for \(\lbrace \tilde{\mbox{{$ h $}}}_{k}\rbrace \) , \(\lbrace \mbox{{$ f $}}_{k,j}^{n_{j}}\rbrace \) , and \(\lbrace \mbox{{$ g $}}_{k,j,q}^{n_{j},n_q}\rbrace \)  is available at the BS by exploiting the channel estimation methods proposed in IRS-aided communications [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, where the obtained solution provides an upper bound on the achievable sum-rate of the users. To tackle the non-convex problem (P1), in the following, we propose an efficient algorithm by exploiting the successive refinement method [7]}, where we optimize each of the \(N\)  reflection coefficients with the other \((N-1)\)  reflection coefficients being fixed in an iterative manner over the \(N\)  reflection coefficients.
